Ethanol obtained from the fermentation of starch from grains or sucrose from sugar cane is being blended with gasoline to supplement petroleum supplies. The relatively oxygenated ethanol increases the efficiency of combustion and the octane value of the fuel mixture. Production of ethanol from grain and other foodstuffs, however, can limit the amount of agricultural land available for food and feed production, thereby raising the market prices of grains and leading to the expansion of agricultural production into forests or marginal lands thereby resulting in ecological damage. Moreover, the intense tillage and fertilization of prime agricultural land for the production of grains can result in excessive soil erosion and runoff or penetration of excess phosphorous and nitrogen into waterways and aquifers. Production of ethanol from lignocellulosic agricultural or woody feedstocks that do not compete with food and animal feed supplies is therefore highly desirous, indeed essential for the large-scale development of renewable fuels from biomass.
Several obstacles currently limit the use of biomass for renewable fuel production. The biomass must be pretreated to extract the sugars, lignins and other components from the starting material. Mild conditions for pre-hydrolysis are desirable because they result in the formation of lower amounts of inhibitory components such as furfural, hydroxymethyl furfural and sugar degradation products such as formic acid. The resulting sugars can be present in the form of monosaccharides such as D-glucose, D-xylose, D-mannose, D-galactose and L-arabinose or as various oligomers or polymers of these constituents along with other lignocellulosic components such as acetic acid, 4-O-methylglucuronic acid, and ferulic acid. Glucose in sugar hydrolysates may repress the induction of transcripts for proteins essential for the assimilation of less readily utilized sugars that are also present in hydrolysates, such as xylose, cellobiose, galactose, arabinose, and rhamnose. In addition, the production of ethanol from glucose can attain inhibitory concentrations even before use of other sugars commences. This results in the incomplete utilization of sugars and sugar mixtures in hydrolysates. Glucose in sugar hydrolysates may also repress the induction of transcripts for proteins essential for the depolymerization of cellulose, cellulo-oligosaccharides, xylan, xylo-oligosaccharides, mannan, manno-oligosaccharides, and other more complex hemicelluloses and oligosaccharides derived from them. These poly- and oligo-saccharides can be present in hydrolysates that have been recovered under mild treatment conditions.
Yeasts such as Saccharomyces cerevisiae and Scheffersomyces stipitis and bacteria such as Escherichia coli, Zymomonas mobilis and Klebsiella oxytoca have been engineered for the production of ethanol from xylose, arabinose, xylo- and cellulo-oligosaccharides since native strains of these organisms are limited either by low production rates, strong preference for utilization of glucose over xylose, susceptibility to inhibitors, susceptibility to microbial or bacteriophage contamination, high requirements for nutrients, or containment regulations due to the expression of transgenes in order to achieve xylose or cellobiose utilization. There remains a need for yeasts that will ferment glucose, xylose, cellobiose and other sugars from lignocellulosic materials at high rates and yields without these drawbacks.